STP ( Spanning Tree Protocol ) | Part 16 | Switching | CCNP | CCNA

Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on bridges and switches. The specification for STP is IEEE 802.1D. The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your network.

The root bridge is selected by manually configuring its bridge priority to a low value. 32768 is the default value out of a range from 0 to 61440. If all switches in a single spanning tree have the same bridge priority, the switch with the lowest MAC address will become the root bridge.

Root Port selection is based on the port having lowest cost to the Root Bridge. When this BPDU receive by any other switch it will add its own port cost (according to the above mentioned values in video). So if BPDU receive by a Fast Ethernet port (100 Mbps) it will calculate path cost to root as 19 (0+19)